Sludge treatment process

ABSTRACT

A system is described for producing small uniform pellets of predetermined size which are substantially free of fines from mechanically dewatered sewage sludge. The system includes a process and apparatus for mixing the dewatered sludge with previously dried, recycled particles of said sludge, drying said mixture in a thermal drier, separating a substantial portion of the dried solids from the drier off gas, clarifying said separated solids to separate the pellets of predetermined size, oversize pellets, and undersize pellets and particles, mechanically crushing the oversize pellets and admixing the crushed oversize pellets with the undersize pellets and particles, and recycling said mixture by mixing said mixture with incoming dewatered sludge to the drier.

This is a division of U.S. Pat. No. 4,829,678, application Ser. No.07/232,640, filed 8/8/88.

This invention relates to an improved system for disposing of sewagesludge, and in particular to a system for efficiently drying andpelletizing said sludge for sale as a plant nutrient, soil conditioner,or bulking agent for fertilizer production. The pellets formed aresubstantially uniform in size and dimensioned to be used in conventionalspreaders.

Waste liquid sludge from a sewage treatment plant may contain over 90%water. Conventional disposal techniques include dewatering, drying anddisposal in, for example, a land fill. This means of disposal, however,is increasingly becoming not feasible due to environmental regulationsand transportation costs.

An alternative, because the sludge contains, when dry, combustiblematerials, is burning the sludge to produce heat. Normally in suchprocedures, the sludge is dried in a drier and then burned in a furnacewherein the off gas is passed through a heat exchanger. See, forexample, U.S. Pat. No. 4,429,643. In the sludge treated in processessuch as that described in the above patent, the wet sludge cake has amoisture content of about 80% as it enters the drier. The moisture isflashed away in the drier and the dried product is pelletized. The offgas from the drier is then mixed with outside air and used as combustiongas in the furnace.

Producing pellets of uniform size has presented a problem in the past.Uniform size pellets are important whether the sludge is to be burned orused as a soil conditioner. In the latter case, uniform size pellets arevery important so that the pellets may be used in conventional spreadingequipment.

In U.S. Pat. No. 3,963,471 to Quintan L. Hampton, a process is describedfor producing uniform pellets wherein the wet sludge cake from amechanical dewatering procedure is mixed with previously dried productpellets and the mixture is then cycled through the drier. It isdescribed that the size of the pellets can be controlled by the amountof material recycled. The recycled pellets then form nucleii for theformation of pellets in the drier as they are surrounded by an adheringlayer of wet sludge cake.

In U.S. Pat. No. 4,040,190 there is described a process wherein therecycled materials are all passed through a crusher before being mixedwith a wet sludge cake. In that patent it is indicated that the crushingdevice may be adjusted for controlling the size of the grains to besupplied to the device and that by crushing to a predetermined size thegrain size of the output pellets can be controlled. However, thisneglects the presence of fines in the recycled product. The entireoutput from the drier is crushed and then a portion diverted to berecycled with the remaining to storage. Therefore, the product wouldinclude fines both from the cyclone separator used for the off gas fromthe drier, and from the crushing of oversized particles wherein saidparticles fracture.

It has been discovered, however, that the output particle size can beregulated within desired limits to substantially eliminate fines fromthe product. According to the system of this invention, the output fromthe drier is initially separated from the fines and routed to a rotaryscreen. The rotary screen then separates the oversized pellets from theregular sized pellets, and the latter from the fines. The airstreamcontaining fines then passes through a cyclone system for removing thefines, and fines from the cyclone system and from the rotary screen arecombined in a recycle bin. The product size pellets are then routed to aproduct bin for sale as a fertilizer, soil conditioner, or the like.Finally, the oversized pellets are crushed and routed to the recyclebin. In this way, product size pellets which may be preferably 4 to 5millimeters in diameter can be provided for sale, use as a fuel or thelike substantially without fines. The system also provides for adeodorizing treatment of the airstream from the cyclone separator beforeemission into the atmosphere. This treatment may include a wet scrubber,a chemical tower, and the like. Any liquid from this treatment is routedback to the head of the plant and added to incoming liquid sludge.

The drier may be heated with conventional means such as oil, natural gasor a wood burner or other suitable fuel may be used to supplement fossilfuel. If desired, the dried sludge may be used as a source of fuel.

Accordingly, it is an object of this invention to provide a system forpelletizing sewage sludge to produce pellets of uniform size useful as asoil conditioner, fertilizer, fuel or the like.

It is another object of this invention to provide a system whereindewatered wet sludge cake is combined with recycled dried sludge pelletsand pelletized in a rotary drier, with the output from said drier beingscreened to separate product size pellets from oversized pellets whichare crushed and from fines whereby the crushed oversize pellets andfines are recycled to be admixed with the dewatered sludge cake.

It is yet another object of this invention to provide a system whereinthe output dried sludge pellets exiting a drier are initially subjectedto a lower velocity centrifugal separation from the airstream and thenscreened to separate oversized, product sized, and fines. Oversizedpellets are then crushed and admixed with the fines for recycling asinput to the drier with dewatered sludge cake.

These and other objects will become readily apparent with reference tothe drawings and following description wherein:

FIG. 1 is a schematic of the system of this invention; and

FIG. 2 is a schematic of a preferred version of the system of thisinvention.

With attention to the drawings and to FIG. 1 in particular, liquidsewage sludge may contain up to 90 to 99% water. According toconventional techniques, the sludge is subjected to flocculation wherebythe sludge particles agglomerate so that when the liquid sewage sludgeenters a mechanical press 10 it can be dewatered down to about 80% waterand then routed to a storage bin 12. The material from bin 12 will beidentified hereinafter as wet sludge cake. As noted, this materialcontains about 80% water.

Wet sludge cake from the storage bin 12 and recycled dried sludge fromthe recycle bin 14 are then routed by conventional conveyor to a mixingpug mill 16. In one embodiment of this invention two pug mills are usedwith a 90° turn. It should be noted that while the material from therecycle bin and the wet sludge cake can individually be conveyed byauger or the like, the mixture in the pug mill cannot. Accordingly, assoon as the product is mixed in the pug mill 16, it enters the drier 18wherein heat is supplied by a conventional furnace 20 or the like. Inthe embodiment shown, the heat from furnace 20 is supplemented by asolid fuel burner 22 which may burn wood chips or the like. In thealternative, burner 22 could be fueled with sludge pellets and/or thecombustion chamber 20 could be fueled with sludge pellets.

Drier 18 is preferably a triple pass rotary drier heated at the entranceto around 800° F. The mixture is intended to travel the length of thedrier, make a 180° turn and travel the length two more times beforeexiting the drier. While several different types of rotary driers couldbe utilized, it is preferable to use the rotary drier described in U.S.Pat. No. 4,558,525.

Pelletized sludge, together with the drier off gases, exit the drier 18and enter a separator can 24. The temperature of this stream is about180° F.

The separator can 24 is a vertically disposed cyclone-type separatorthat uses gravity and centrifugal force to remove about 85 to 90% of thesolids from the gas stream. The gas stream then exits the separator intoa cyclone 26 where the fines are removed and the fines then exit cyclone26 at exit port 28 and are added to the recycle bin 14.

Cyclone 26 could be a conventional cyclone system, but it is preferredto use the separator described in U.S. Pat. No. 4,357,152 for moreefficient removal of the solids from the gas stream. Solids fromseparator 24 then enter screen system 30. Preferably, screen system 30is a rotary screen which also acts as a cooler. In the preferredembodiment of this invention, 1/8 inch and 3/32 screens are provided.Material larger in diameter than 1/8 inch is classified as oversize.Particles in diameter between 1/8 inch and 3/32 inch are the product andare routed to a product bin 32. The oversize particles are then routedto a conventional crusher 34 which may be a roller or the like. Thecrushed particles are then routed to the recycle bin 14. The finespassing through the 3/32 inch screen then exit the screen and are alsorouted to the recycle bin. As shown in FIG. 1, in the event of a needfor more recycle material, product may bypass the product bin and berouted to the recycle bin.

Finally, the airstream exhausted from cyclone 26 passes preferablythrough a scrubber 36, a packed tower 38, and a chemical tower 40 beforebeing exhausted to the atmosphere through stack 42. This procedure willeliminate any solids remaining beyond that permitted by air pollutionregulations as well as any noxious odors. The liquid from the airtreatment system 36, 38, 40 and 42 is then returned to the head of thewaste water treatment plant to be combined with the liquid sludge beingadmitted to press 10 for recycling.

During normal operation the product pellets have an optimum size ofabout 4 to 5 millimeters and constitute about 10% of the solids passingthrough the system.

As previously indicated, the entrance temperature at the rotary drier isabout 800° F. The temperature of the airstream exiting the drier isabout 180° F. After cooling in the rotary cooler screen system 30, therecycle materials in the recycle bin are at a temperature of about 140°F.

With attention to FIG. 2, FIG. 2 represents a slight modification of thesystem of FIG. 1. The source of heat for drier 50 is a conventionalfurnace system 52 which utilizes a conventional fuel such as oil. Wetsludge cake from a storage bin 54 is routed to a first pug mill 56 wherethe wet cake is mixed with dried pellets from the recycle bin 58. Themixture from mill 56 then drops into a second pug mill 60 and makes a90° turn as shown to promote mixing.

The mixture then enters drier 50 in front of the outlet from furnace 52.Drier 50 is preferably a triple pass drier as described above and shownin U.S. Pat. No. 4,558,525. After passing through the drier the streamincluding solids enters separator can 62. As described above, separatorcan 62 is a low velocity cyclone type separator. A commercial versionmay be obtained from Progressive Development Incorporated of Milwaukee,Wis. The air and fines from separator can 62 then enter either a pair ora single cyclone type separator 64 which are preferably the devicesdescribed and claimed in U.S. Pat. No. 4,357,152. The fines from thecyclone 64 are routed from additional cyclone separator(s) 66 to therecycle bin and the airstream returned to cyclone 64. The airstreamexiting cyclone 64 then is treated in an odor controlling tower 68,which may be of conventional design, before the air is exhausted to theatmosphere.

The solids from separator can 62 then are routed to a cooler screeningdevice 70. Preferably as described there are two screens in the coolerseparator device. The first screen in this version separates outparticles less than 3/32 inch in diameter and these particles are routeddirectly to the recycle bin 58 as shown. The second screen separates theproduct which has preferably a diameter of between 1/8 inch and 3/32inch and that product is routed to storage 72 for eventual sale.

The oversize particles remaining then enter crusher 74 and areeventually rerouted to the recycle bin 58 for recycling through pugmills 56 and 60 to drier 50, with wet sludge cake.

In summary, the system of this invention provides a means foreliminating fines and undersized material from a product so that driedpelletized sewage sludge may be marketed in truly uniform pellet sizes.In addition, by crushing only the oversized particles, and by recyclingthe crushed material, the size of the product formed can be strictlyregulated. The system of this invention also obviates the need forpassing fines and product size pellets through a crusher wherein eventhe product size pellets could be crushed against each other. The systemof this invention combines a low velocity separator which removes up toabout 85-90% of the solids exiting the drier and a screen device forsolids separation so that the smaller particles and oversized particlesafter crushing can be recycled.

While there have been described and illustrated several specificembodiments of the invention, it will be clear that variations in thedetails of the embodiments specifically illustrated and described may bemade without departing from the true spirit and scope of the inventionas defined in the appended claims.

What is claimed is:
 1. A process for drying mechanically dewateredsewage sludge to produce pellets of a predetermined small and uniformsize comprising the steps of:providing a first and a second pug mill,each having an inlet and an outlet, said second pug mill being disposedbelow said first pug mill and at about a 90° angle thereto; mixing saidsludge with recycled, previously dried sludge particles and pellets insaid first pug mill; dropping said mixture into said second pug mill;mixing said material further in said second pug mill; providing a rotarydryer having a inlet and an outlet, the inlet being disposed adjacentthe outlet from said second pug mill; dropping said mixture from saidsecond pug mill into the inlet of said rotary dryer and drying saidmixture to form pellets and particles thereof; conveying the driedpellets and particles and drier off gas to a low velocity gas/solidsseparator and separating and collecting the pellets and a substantialportion of the particles; clarifying the collected pellets and particlesand separating and collecting pellets of a predetermined size, theundersize pellets and particles and oversize pellets constitutingrecycle material; mechanically crushing the recycle materials andcollecting the crushed pellets and undersize particles and pellets; andrecycling the collected undersize pellets and particles by mixing saidrecycle materials with said incoming sludge in said first pug mill. 2.The process of claim 1 wherein the step of clarifying further includesproviding a first screen which will pass pellets and particlestherethrough of a predetermined size or smaller and providing a secondscreen which will pass only pellets and particles smaller than thepredetermined size whereby the pellets of a predetermined size may becollected as product.
 3. The process of claim 2 wherein the step ofmechanically crushing includes mechanically crushing only the oversizepellets retained on said first screen.
 4. The process of claim 1 whereinsaid drier is a triple pass rotary drier.
 5. The process of claim 4wherein the step of clarifying further comprises cooling said collectedpellets and particles.
 6. An apparatus for drying dewatered sewagesludge to produce pellets of a predetermined small and uniform sizecomprising:means for mixing said sludge with recycled, previously driedsludge particles and pellets including first and second pug mills, eachhaving an inlet and an outlet said second pug mill being disposed belowsaid first pug mill and substantially at a 90° angle thereto; rotarythermal drying means downstream of said second pug mill for receivingsaid mixture and drying said mixture to form pellets and particlesthereof; gas/solids separator means downstream of said drying means forreceiving the dried pellets and particles and off gas from said dryerand separating a substantial portion of said pellets and particles fromsaid off gas; clarifying means downstream of said separator forreceiving said separated pellets and particles and for separatingtherefrom the pellets and particles of a predetermined size, theremainder constituting recycle materials; mechanical crushing meansdownstream of said clarifying means for receiving and mechanicallycrushing said recycle materials; and storage bin means and conveyingmeans for conveying the crushed recycle materials to the storage binmeans whereby the contents thereof may be recycled by mixing the samewith incoming dewatered sludge to be admitted to said first pug mill andfrom said first pug mill to said second pug mill and from there intosaid drying means.
 7. The apparatus of claim 6 wherein the clarifyingmeans comprises a first screen for separating pellets above apredetermined size and a second screen for retaining pellets of saidpredetermined size; means for conveying the pellets retained on thefirst screen to the mechanical crushing means.
 8. The apparatus of claim7 wherein said clarifying means further comprises means for cooling thepellets and particles.
 9. The apparatus of claim 8 wherein said dryingmeans includes a rotary triple pass dryer.